Radio Frequency (RF) Wireless Local Area Network (WLAN) technology is evolving into the EHF or “extremely high frequency” band from 30 to 300 GHz. This band, also called the millimeter band, covers radio waves with wavelengths from one to ten millimeters. This band extends from 30-300 GHz, and some applications focus on the 60 GHz ISM (industrial, scientific and medical) radio band.
Specialized RF design techniques are used when designing circuits for the millimeter band. Excessive PCB (printed circuit board) losses constrain RF signal routing to very short distances, limiting the size of antenna arrays. RF cables are also typically not used, due to losses. Power amplifier (PA) technology at 60 GHz is currently limited to 20 dBm, 16 dB lower than commercial 6 GHz WLAN PAs. Finally, first meter losses at 60 GHz are 20 dB greater than seen at 6 GHz.
Some RF solutions at 60 GHz are designed for fixed point-to-point applications, where high gain horn or horn-fed parabolic antennas are employed. In these cases, the small wavelength enables high gain antennas of 40-50 dB to be realized to support links of several km. However, these solutions cannot easily be used for point-to-multipoint Wireless LAN applications as a single radio transceiver must provide wide-angle coverage.
Other WLAN solutions targeted for the 60 GHz band employ active antenna chips with multiple transceivers. These solutions are intended for beamforming, with up to 32 active RF elements each transmitting 3-5 dBm. The combined solution achieves an appreciable gain (+36 dBm equivalent isotropically radiated power (EIRP)) if all elements are used, but is unable to achieve 360 degree coverage with this solution which assumes array antennas, and beamforming gain, since the combined antenna arrays are less than 4 cm2.
In millimeter wave applications, highly directional narrow band antennas are used due to high loss at high frequencies. Thus, when hemispherical coverage is needed, as is the case for a wireless personal area network (PAN), for example, multiple antennas are typically needed. Consequently, multiple antenna feed connections are needed. However, difficulties in printed circuit board (PCB) routing, switching and power amplification lead to designs that include high antenna array gain and active element count.
Array gain can be improved simply by increasing the gain of the individual antenna elements of the array. However, the high antenna gains tend to further restrict the directional beamforming of the combined transceiver system that includes the antenna array. For example, a 20 dBi (decibel isotropic) flat panel antenna has a typical beam width of 10 degrees in elevation and azimuth. An 8 dBi patch antenna has a typical beam width of 65 degrees in elevation and azimuth. The base element used in each element of the array determines the overall gain of the array, while limiting the beamforming capabilities. Using the following formula,Effective beamforming gain=Fixed element gain+20*log(number of elements),the beam forming gain can be computed. For example, starting with an 8 dBi base element with a coverage angle of 65°×65°, the effective beamforming gain with 32 active elements is 8 dBi20*log(32)=38 dBi. Allowing 2 dB for implementation and track losses, this system would achieve 36 dBi gain along a bore sight of the antenna array, and up to 30 dBi gain at the coverage edges. This solution would not achieve significant gain past the defined coverage angle, and hence, is not a good solution for indoor omni-directional coverage.
WLAN RF designers and chip manufacturers consider solutions which follow a conventional WLAN Wi-Fi design approach using surface mount, highly integrated media access control (MAC), baseband, and RF chipset solutions to enable low radio cost products to be realized. These designs utilize printed circuit board (PCB) panel antennas—effectively fixed direction antennas, and are limited by the RF coverage of these antennas.
Referring to FIG. 1, typical microwave WLAN RF switches for Wi-Fi and other radio protocols are designed for microstrips where the signal “A” 12 is routed as a top layer of a PCB 10 where the ground layer “D” 14 is routed at a defined dielectric “C” 16 distance below the microstrip. As a result, surface mount RF switches are typically employed for lower-frequency applications, but are unsuitable for millimeter wave applications due to high losses. For at least these reasons, switchable microstrips are unsuitable for switchable routing of millimeter wave signals to omni-directional antenna configurations.